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Preparation, characterization of modified wheat residue and its utilization for the anionic dye removal
Desalination 267 (2011) 193–200
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation, characterization of modified wheat residue and its utilization for the anionic dye removal Qian-Qian Zhong, Qin-Yan Yue ⁎, Qian Li, Xing Xu, Bao-Yu Gao Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 27 April 2010 Received in revised form 15 September 2010 Accepted 15 September 2010 Available online 12 October 2010 Keywords: Sorption Kinetics Equilibrium isotherm Dye Modified wheat residue (MWR)
a b s t r a c t The utilization of modified wheat residue (MWR) as sorbent to remove the anionic dye (Reactive Red-24, RR24) from aqueous solution was studied. MWR was prepared and characterized by specific surface area, SEM, zeta potential, FTIR and elemental analysis. Sorption experiments were carried out as a function of sorbent dosage, pH, contact time and concentration of dye. Results indicated that a mass of amine groups were grafted into the framework of MWR. It was shown that pseudo-second-order kinetic equation could best describe the sorption kinetics. The equilibrium sorption data were well represented by the Langmuir isotherm equation. The maximum sorption capacity of MWR for RR-24 was 200.0 mg/g, which showed higher capacity than those of previous work and a similar capacity compared to those of commercial activated carbon. The results indicated that MWR could be employed as an excellent and low-cost sorbent for removal of anionic dye from aqueous solution. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The textile industry produces large amount of colored effluents, in which dyes are highly visible and can be carcinogens and toxic to aquatic life in water. Reactive dyes have been identified as the most problematic compounds in textile dye effluents, as they tend to pass through conventional treatment systems unaffected. Conventional biological treatment processes are not very effective for their high solubility and non-biodegradability [1,2]. Therefore many methods, such as coagulation–flocculation, chemical oxidation, membrane processes, and adsorption [1,3–7], are used for the removal of the dyes. Of all these, activated carbon is the most commonly used sorbent for the treatment of dye bearing wastewaters. However, this process is proved to be uneconomical due to the high cost of activated carbon and also the additional cost involved in regeneration [8]. Therefore, further development of sorbents has been investigated, which focuses on the research of sorbents prepared from agricultural residues (AR), including bagasse [8], rice husk [9], orange peel [10], pine sawdust [11], coconut husk [12], pomelo peel [13] and apple pomace [14]. In North China, wheat is a very common and abundant crop and its by-product wheat residue (WR) is being considered as a significant waste disposal problem nowadays. The idea of converting wheat residue into a sorbent is based on the predominant composition of cellulose (32.1%), hemicellulose (29.2%), lignin (16.4%) and extrac-
⁎ Corresponding author. Tel.: + 86 531 88365258; fax: + 86 531 88364513. E-mail address: [email protected] (Q.-Y. Yue). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.025
tives (22.3%) in WR [15–17]. Due to a large amount of easily accessible hydroxyl groups that exist in cellulose, hemicellulose and lignin structures, a series of chemical reactions, such as condensation, etherification and copolymerization can easily happen [17,18]. In the present work, a new sorbent based on WR is present, which comprises first of etherification at alkaline condition, then an introduction of amine groups into the framework of WR. MWR bearing amine groups was characterized in relation to its physicochemical structure and then used as sorbent for the removal of anionic dye (RR-24) from aqueous solution. Batch studies were performed to evaluate the effect of various experimental parameters on the removal of RR-24. In addition, kinetic studies were carried out taking the initial dye concentration into account. The sorption capacities of MWR for RR-24 were investigated by the equilibrium isotherms. Thermodynamics of sorption process were studied and the changes in Gibbs free energy, enthalpy and entropy of sorption were also determined. 2. Materials and methods 2.1. Preparation of MWR Biomass WR, collected from Liao Cheng, Shandong, China, was washed with tap water followed by distilled water, and oven dried at 105 °C for 24 h, and then sieved, finally the particles ranging from 150 to 380 μm were selected for further chemical modification. Six grams of WR was dispersed in 100 mL of 15%NaOH (w/w) in a 500 mL three-neck round bottom flask and stirred for 1 h at room temperature followed by adding 40 mL of epichlorohydrine and the mixture was stirred for 3.5 h at 30 °C. The reaction product was
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CI
washed with distilled water to remove residual chemicals, and dried at 60 °C in a vacuum drier for 12 h. The dried product was then reacted with 50 mL 40% dimethylamine solution (w/w) for 5 h at 30 °C. The reaction product was washed twice with distilled water until the eluant was neutral and then dried at 60 °C in a vacuum drier for 12 h. The final product MWR was obtained and used in all sorption experiments. The chemical reactions using cellulose as a starting material are shown in Fig. 1.
N
SO3Na OH
NH
N
N
N N NH SO3Na
NaO3S
2.2. Preparation of dye solution
Cl
Fig. 2. Chemical structure of RR-24 (chemical formula: C25H14N7Cl2O10S3Na3 molecular weight: 808.5).
RR-24, obtained from Bin Zhou Dye Printing Co. (Shandong, China), in commercial purity, was used without further purification. The chemical structure of RR-24 is shown in Fig. 2. The dye stock solutions were prepared by dissolving accurately weighted dyes in distilled water to the concentration of 1000 mg/L and subsequently diluted when necessary.
and coated with platinum in vacuum evaporator before observation. The accumulation voltage and current were 3.0 kV and 10 μA, respectively. Zeta potential measurements were carried out using a microelectrophoresis apparatus (JS94H, Shanghai Zhongchen Digital Technical Apparatus Co., Ltd, China) to determine the zeta potentials of MWR and WR. FTIR spectra were recorded on an Avatar 370 spectrometer (Thermo Nicolet, USA) to investigate the functional groups present in MWR. For the spectra, 5 mg of samples was encapsulated in 400 mg of spectroscopically pure KBr and the specimens were pressed into small translucent discs. IR spectra were obtained by averaging 60 scans from 4000–400 cm− 1 region at 2 cm− 1 resolution.
2.3. Characterization of MWR Specific surface area measurements were performed with an automatic BET surface area analyzer (Model F-Sorb 2400, Beijing Jinaipu Technical Apparatus Co., Ltd, China). The detection limit of this instrument, using N2, is 0.01 m2 g− 1. SEM of the sample was obtained by Scanning electron microscope JSM-7600 F, JEOL, Japan). The samples were mounted on metal grids
OH H
OH
CH2OH O
O
H O CH2OH
H
OH
CH2ONa
H
O
OH
OH
CH
CH2
OH
CH H
H OH CH2
CH2O CH2 CH
CH2
+
O
CH3
(Dimethylamine)
HN
OH
CH3
O
OH OH
H
CH2O CH2
CH
O O
H O H
CI
(Epichlorohydrine)
CH2O CH2 H
O
OH
CH2
O H
H H
+
O
OH
O O
H
H
H
O CH2ONa
H
NaOH
(Cellulose)
O O
+
O
OH
OH H
OH
H
H
H
OH
H H OH
+ CH2OCH2 CH CH2 N H CI
H
CH2
+ NH CI
O
OH CH3 CH3
Fig. 1. Surface chemical modification of cellulose.
CH3 CH3
Q.-Q. Zhong et al. / Desalination 267 (2011) 193–200
The nitrogen content of MWR was measured by element analyzer (Elementar Vario EL III, Germany) to evaluate the grafted amine groups in the MWR.
195
The amount of adsorbed dye qt (mg/g) at different time t, was calculated as follows: qt =
ðC0 −Ct ÞV m
ð1Þ
2.4. Sorption experiments Sorption experiments for MWR were carried out in a batch equilibrium technique. The pH of the dye solution was adjusted using HCl or NaOH (0.1 mol/L). The sorption of dye was performed by shaking a predetermined amount of MWR in a 100 mL synthetic dye solution (with known initial dye concentration) at 140 rpm on a horizontal shaker. The solutions initial pH value (pH = 4.6) was used throughout all sorption experiments. After a given contact time for sorption, the solution was filtered and then analyzed quantitatively. The residual dye concentration in the clear supernatant was then measured with UV–visible spectrophotometer (model UV754GD, Shanghai) at the wavelength corresponding to a maximum absorbance, λmax, which is 534 nm for RR-24.
where C0 (mg/L) and Ct (mg/L) are the initial concentration of dye and the concentration at time t, respectively. V (l) stands for the volume of solution, and m (g) is the weight of the sorbent. The dye removal efficiency, η, was calculated by the following equation: ηð%Þ =
C0 −Ct × 100 C0
ð2Þ
3. Results and discussion 3.1. Characterization Data of specific surface areas of MWR and WR are only 4.8 and 6.1 m2 g− 1, respectively. A slight decrease in specific surface area of MWR is observed after the chemical modification; this result indicates that the extractives have been gotten rid of from the surface of MWR after the chemical modification. The low specific surface area of MWR illustrates the fact that MWR does not have a similar porous structure in active carbon (specific surface area higher than 500 m2 g− 1), and therefore, surface sorption will be absent in the potential sorption mechanism for RR-24 sorption onto MWR. The SEM images of the structure of WR and MWR (Fig. 3) show that the surface of MWR is smoother than that of WR, which validates the lower specific surface area of MWR in comparison with that of WR. Similar results were reported in the works of Xu and Wang for the preparation of anionic absorbents from agricultural residues [16,19]. The zeta potentials of WR are in the range of −35 to −30 mV. The great negative zeta potential of WR can be attributed to the large amounts of hydroxyl and carboxyl groups existing in WR. In contrast with the WR, the zeta potential of MWR is in the range of −10 to 0 mV; this result indicates the existence of positive-charge functional groups in the framework of MWR, which results in the significant decrease in the negative charge of MWR. The FTIR spectra of WR and MWR are presented in Fig. 4. The band intensity near 3390 cm− 1 indicates a mass of O–H stretching vibration, and the band position at 2900 cm− 1 is associated with C– H stretching vibration [20]. The bands observed in the 1625 cm− 1 is due to O–H bending vibration of water molecules, which is consistent
(a)
Transmittance (a.u.)
(b)
WR MWR
500
1000
1500
2000
2500
3000
Wavenumber(cm-1) Fig. 3. SEM images of WR (a) and MWR (b).
Fig. 4. FTIR analysis of WR and MWR.
3500
4000
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with the work of Marangonia [7]. The FTIR analysis of MWR displays the relative changes in the structure compared with that of WR. The band at 1630 cm− 1 is attributed to the N–H stretching [21], and an intense vibration is observed in the band at 1410 cm− 1, which corresponds to characteristic for C–N stretching vibration. These obviously come from amine groups grafted into the structure of the MWR. Similar results were also observed in Mishra and Gurgel's work [21,22]. Table 1 displays the elemental changes of carbon, hydrogen, and nitrogen in MWR in comparison with those of WR. A slight increase is observed in the carbon content and hydrogen content. Nitrogen content of MWR, however, increases significantly from 0.35% to 1.04%, indicating a three fold increase compared to WR. Therefore, it is possible to conclude that the reactions proceeded efficiently and amine groups have been anchored onto the MWR. Based on the characterization of MWR mentioned earlier, it is clear that the MWR has been grafted with a mass of functional groups which would be beneficial to the sorption of anionic dyes from solution.
Dye removal efficiency(%)
100
80
60 50mg/L dye + MWR 100mg/L dye + MWR
40
150mg/L dye + MWR 200mg/L dye + MWR 100mg/L dye + WR
20
0 0.4
0.8
1.2
1.6
2.0
Sorbent dosage(g/L) Fig. 5. Effect of sorbent dosage on sorption of RR-24 by MWR and WR (T = 26 °C, t = 4 h, initial pH).
3.3. Effect of pH on dye removal The pH of the solution plays an important role in the sorption capacity of sorbate molecule largely due to its influence on the surface characteristics of the sorbent and ionization/dissociation of the sorbate molecule [10]. Fig. 6 presents the effect of pH on dye removal using MWR. It is evident that dye uptake is higher at lower pH and as the pH of the solution increases, dye uptake decreases sharply. The RR-24 dye is reactive dyestuff, which contains –SO− 3 groups in its structure. Under acidic pH, the protonation of –SO− 3 groups enhances, and then more –SO3Na of dye molecule exists in the form of –SO3H, which can react with the amidocyanogens of MWR to + − − form –NH+ 2 SO3 or = NH SO3 [23]. As a result, the sorption of RR-24 onto MWR is significantly enhanced. However, as the pH of the system increases, the protonation of –SO− 3 decreases, and consequently – SO3H groups are reduced, resulting in the decrease of interaction between the dye molecules and MWR. Moreover, lower sorption of the anionic dye at alkaline pH is provable attributed to the abundance of OH− ions, which will compete with the dye anions for the sorption sites [10,24,25]. Similar observations have been reported by previous work for sorption of reactive dyes on activated carbon prepared from sugarcane bagasse pith, coir pith and orange peel [10,26,27].
The relationship between the initial and final pH for dye sorption indicates that MWR has a high pH buffering nature (Fig. 6). At lower pH, due to more H+ combined with –SO− 3 groups, the final pH elevates, resulting in the buffering of H+ by OH−. Similarly under alkaline pH, the final pH decreases compared with the initial pH, because of the sorption of OH− from the solution by MWR.
3.4. Effect of contact time and initial concentration The effects of initial dye concentration and contact time on sorption of RR-24 onto MWR are investigated in Fig. 7 (a). As shown in Fig. 7 (a), the amount qt of RR-24 sorbed onto MWR increases from 41.5 to 118.9 mg/g as the initial concentration of dye increases from 50 to 150 mg/L, attributed to the increase in the driving force of the concentration gradient with the increase in the initial dye concentration [28]. The contact time is an important parameter in the wastewater treatment by sorption. It is observed that the contact time increases with an increase of initial concentration of the dye. The results show that the contact time needed for RR-24 solutions with initial concentrations of 50–100 mg/L to reach equilibrium are within 60 min. For RR-24 solution with initial concentration of 150 mg/L, equilibrium time of 240 min is required. It may be due to the increase
100
12
80
10
8
60
6
Final pH
The effect of sorbent dosage on RR-24 removal is shown in Fig. 5. It is observed that the removal efficiency of dye increases up to a maximum efficiency as the dosage of MWR increases, probably due to the increased availability of more sorption sites with the increase in MWR dosage. For the concentration in the range of 50–150 mg/L dye solutions treated using 1.2 g/L MWR, all the dye removal efficiencies exceed 92.2%; meanwhile, for dye concentration of 200 mg/L, 95.7% of the removal efficiency is obtained using MWR dosage of 2.0 g/L. Fig. 5 also illustrates the RR-24 removal capacity of WR. It is clearly seen that RR-24 is hardly absorbed by WR. The results probably indicate that a chemical modification is required to introduce some functional groups into the structure of WR for the dye removal. A similar conclusion was observed in the adsorption of phosphate onto modified wheat straw prepared by triethylamine [17].
Dye removal efficiency(%)
3.2. Effect of sorbent dosage on dye removal
40 4 20 2
Table 1 Change of element content of WR and MWR.
0
Materials
N (%)
C (%)
H (%)
WR MWR
0.35 1.04
41.11 43.58
6.10 6.64
2
4
6
8
10
12
initial pH Fig. 6. Effect of initial pH on decolorization and final pH (MWR dosage 1.2 g/L, initial RR24 concentration 100 mg/L, t = 26 °C, t = 4 h).
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197
(b) pseudo first-order
(a)
6
120 4 100
ln(qe-qt)/mg g-1
qt/ mg g-1
2 80
60
40
0 -2 -4
50mg/L
50mg/L 80mg/L
80mg/L
20
-6
100mg/L
100mg/L
150mg/L
0
50
0
100
150
200
150mg/L
-8
250
0
50
100
t (min)
150
200
t (min)
(c) pseudo second-order
(d) intra-particle equation
6
120 50mg/L
5
80mg/L
100
150mg/L
4
qt/ mg g-1
t/qt /min g mg-1
100mg/L
3
80
60
2 40 50mg/L
1
80mg/L
20
100mg/L 150mg/L
0 0
50
100
150
200
0
250
0
2
4
6
8
10
12
14
16
t1/2 (min)
t (min)
Fig. 7. Effect of contact time on the uptake of RR-24 by MWR at different initial concentrations (a) and kinetics models (b, c, d) (T = 26 °C, MWR dosage 1.2 g/L, initial pH).
in dye concentration, the competition for the active sorption sites increases and it takes longer time to reach equilibrium [29]. It is observed from Fig. 7 (a) that the sorption process of RR-24 onto MWR mainly consists of two stages: initial rapid stages and gradual sorption stages. The high sorption rate during the initial period is due to a number of available sorption sites of the bare surface of sorbent. As these sites become progressively covered, the rate of sorption decreases [30]. 3.5. Sorption kinetics In order to investigate the controlling mechanisms of sorption process such as chemical reaction, mass transfer and diffusion control, several kinetic models are required to analyze the experimental data [10].Three kinetic models, namely pseudo-first-order, pseudo-second-order and intra-particle diffusion models are used for the sorption of RR-24 onto MWR. The pseudo-first-order equation [30,31] is given as Eq. (3): ln ðqe −qt Þ = ln qe −k1 t
ð3Þ
where qe and qt (mg/g) are the amounts of dyes sorbed per unit weight of sorbent at equilibrium and time t (min), respectively;
k1 (min− 1) is the rate constant of pseudo-first-order sorption. The pseudo-second-order kinetic rate equation [32,33] is expressed as Eq. (4): t 1 t = + qt qe k2 qe2
ð4Þ
where k2 (g/(mg min)) is the equilibrium rate constant of pseudosecond-order sorption. The initial rate of intra-particle equation can be described as Eq. (5) [17,33]: qt = kp t
0:5
+C
ð5Þ
where kp (g mg− 1 min− 0.5) is the intra-particle rate constant, which can be evaluated from the slope of the linear plot of qt versus t0.5; C is the intercept, related to the thickness of the boundary layer. The kinetic parameters at different initial concentrations are calculated and correlation coefficients (R2) are evaluated as shown in Fig. 7 (b, c, d) and Table 2. Results shown in Fig. 7 indicate that the pseudo-second-order kinetic model can be applied for the entire sorption period. The values of R2 (Table 2) of pseudo-second-order model for RR-24 sorption are satisfactory (N0.9989) and followed by those of modified pseudo-
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Table 2 Kinetic parameters for sorption rate expressions. C mg/L 50 80 100 150
qe expt mg/g
Pseudo-first-order
41.66 66.61 82.98 118.8
Pseudo-second-order
Intra-particle diffusion
k1 min− 1
qe,cal mg/g
R2
k2 g/mg min
qe,
0.1103 0.0389 0.02978 0.02106
17.86 23.47 32.09 76.96
0.9661 0.9690 0.9424 0.9706
0.02338 0.005319 0.002721 0.000647
41.81 67.52 84.67 124.2
cal
mg/g
R2
kp mg/(g min)
C
R2
0.9999 0.9999 0.9999 0.9989
7.657 7.998 9.969 10.87
9.177 21.26 17.75 16.79
0.9062 0.8263 0.9329 0.9448
Where qe,expt is experimental value, qe,cal is calculated value.
first-order equation and intra-particle diffusion equation, respectively; this result indicates that the pseudo-second-order kinetic model generates the best agreement with the RR-24-MWR sorption system. Similar kinetic result was reported for the sorption of Direct N Blue106 onto activated carbon from orange peel [10]. According to Eq. (5), if the plot of qt versus t0.5 passes through the origin, intra-particle diffusion will be the rate-controlling step [10]. However, it is not the case in Fig. 7 (d), and therefore, intra-particle diffusion is not the sole rate-limiting step but some degree of boundary layer diffusion also controls the sorption [23]. Moreover, other sorption kinetic processes occur simultaneously during the RR24–MWR interactions and contribute to the sorption mechanism [10,17]. 3.6. Sorption isotherms The sorption isotherm of RR-24 onto MWR is illustrated in Fig. 8. The batch experimental curves are applied to three sorption isotherm models which are Langmuir, Freundlich, and Dubinin–Radushkevich (D–R).The models are represented mathematically as follows [34,35]: Langmuir equation: 1 1 = + qe qmax
1
1 qmaw KL Ce
ð6Þ
Freundlich equation: ln qe = ln KF +
1 ln Ce n
ð7Þ
D–R equation: 2
ln qe = ln qm −βε
ð8Þ
where qe (mg/g) and Ce (mg/L) are the sorbate equilibrium concentrations in solid and the liquid phases; qmax(mg/g), the
1 E = pffiffiffiffiffiffiffiffiffiffiffiffi −2β
ð9Þ
The theoretical parameters of isotherms along with regression coefficient are listed in Table 3. The Langmuir model yields the best fit (R2 = 0.989–0.995) with compared to the other isotherm models, which shows the homogeneous nature of the sorbent. Based on the Langmuir equation, the value of qmax increases with the temperature increasing, and reaches 200.0 mg/g at 55 °C, which confirms that the sorption process for RR-24 onto MWR is an endothermic reaction. The n values of Freundlich model are in the range of 2–10, indicating a favorable sorption process [34]. The E values of D–R isotherm obtained are lower than 8 kJ/mol; this indicates that the physical interaction is the process involved for adsorption of RR-24 onto MWR, similar observation was reported for sorption of Cr (VI) onto carbonaceous adsorbents prepared from waste biomass [35]. Table 4 lists the comparison of maximum sorption capacity of some dyes on various sorbent. Although the published values are obtained under different experimental conditions, they still will be useful as a criterion for comparing the sorption capacities. As can be seen from Table 4, the maximum sorption capacity of MWR for RR-24 is higher than those in most of previous studies and similar to those of commercial activated carbons; this indicates that MWR has a certain potential for practical application in dye removal from wastewater. 3.7. Sorption thermodynamic In engineering practice, thermodynamic parameters are crucial and must be taken into consideration in order to determine the spontaneity of a process, including the free energy changes (ΔG0), standard enthalpy changes (ΔH0) and the entropy changes (ΔS0) associated with the sorption process [30]. It can be calculated by the following equations [43]:
200
150
qe(mg/g)
monolayer capacity of the sorbent; KL (L/mg) is a constant of the Langmuir isotherm. KF (L/g) is Freundlich constant, and 1/n is the heterogeneity factor. qm (mg/g) is the theoretical saturation capacity in D-R equation; β the constant related to the sorption energy (mol2/ kJ2); and ε the Polanyi potential is equal to RT ln(1 + 1/Ce); E (kJ/mol) provides information about chemical and physical sorption and can be determined according to Eq. (9).
0
ΔG = −RT ln K0
ð10Þ
100 25°C 40°C 55°C
50
0
ln K0 =
0
100
200
300
400
500
600
Ce(mg/L) Fig. 8. Sorption isotherms of RR-24 onto MWR (MWR dosage 1.2 g/L, initial pH).
ΔS0 ΔH 0 − R RT
ð11Þ
where KL is Langmuir equilibrium constant (L/mol), R is the universal gas constant, 8.314 J mol− 1 K− 1, T is the absolute temperature. The enthalpy change is determined by plotting lnK0 versus 1/T. K0 is the thermodynamic equilibrium constant, calculated with respect to temperature using the method of Khan and Singh [43] by plotting ln (qe/Ce) versus qe and extrapolating to qe = 0.
Q.-Q. Zhong et al. / Desalination 267 (2011) 193–200
199
Table 3 Parameters obtained from Freundlich, Langmuir and D–R models. T (°C)
Langmuir isotherm qmax (mg/g)
KL (L /mg)
R2
KF (L/g)
Freundlich isotherm n
R2
qm (mg/g)
Dubinin–Radushkevich isotherm β × 102 (kJ2/mol2)
E (KJ/mol)
R2
25 40 55
172.4 188.7 200.0
0.09236 0.07715 0.07205
0.9953 0.9925 0.9893
70.60 74.38 78.19
6.897 6.734 6.729
0.9092 0.9510 0.9745
140.2 154.4 160.9
7.42 5.48 4.35
2.596 3.021 3.390
0.9033 0.9193 0.8629
Table 4 Comparison of the qe of some dyes on various sorbents. Dyes Reactive Reactive Reactive Reactive Reactive RR-24 RR-24 RR-24 RR-24 RR-24 RR-24 RR-24
red 195 red 2 red 194 red 194 red 120
Sorbent
qmax (mg/g)
Reference
Wheat bran Metal hydroxide sludge Sorel's cement Pine-fruit shell Coconut tree flower carbon Aluminum magnesium mixed hydroxide Chitosan Sludge activated carbon Modified resin Commercial activated carbon (basic) Commercial activated carbon (acidic) MWR
119.1 62.5 120.89 20.8 181.9 657.5 245 37.3 177.1 242 ± 7 163 ± 2 200.0
[32] [36] [37] [38] [39] [34] [40] [41] [2] [42] [42] This work
The thermodynamic parameters are listed in Table 5. The ΔG0 is negative and changes with the rise in temperature, which suggests that the sorption process is spontaneous in the nature and the spontaneity increases with the rise in temperature. ΔH0 (1.704 kJ/ mol) of the system is negative and smaller than 40 kJ/mol, indicating that the sorption of RR-24 onto MWR is endothermic and physical in nature [21], which is also verified by the sorption isotherm studies. The positive value of ΔS0 implies increased randomness at the solid– solution interface with some structural changes during sorption process and an affinity of the sorbent for dye [37,44].
4. Conclusions This study shows that MWR is a promising sorbent for removal of anionic dye, RR-24, from aqueous solution. The ratios of RR-24 sorbed keep above 95% over a range from 50 to 200 mg/L of dye concentration when 2.0 g/L of sorbent is used. A decrease in the pH of solutions leads to a significant increase in the removal of dye RR-24 on MWR. The pseudo-second-order model provides the best correlation of the experimental data and intra-particle diffusion is involved in the sorption process, but it is not the sole rate-limiting step. The isothermal data indicate that the Langmuir model provides much closer fittings than those of Freundlich and D–R models. The saturated sorption capacity of MWR for RR-24 is 200.0 mg/g at 55 °C. Negative ΔG0 indicates spontaneous sorption of RR-24 dye onto MWR at the studied temperatures. The enthalpy change ΔH0 (1.704 kJ/mol) of the system is positive and smaller than 40 kJ/mol, indicating that the sorption of RR-24 onto MWR is endothermic and physical in nature. This work shows that the MWR can be used as an inexpensive and effective alternative sorbent for the purification of wastewater to remove reactive dye from dye wastewater. Table 5 Thermodynamic parameters for the sorption of RR-24. T (K)
Ko
ΔG0 (kJ/mol)
ΔH0 (kJ/mol)
ΔS0 (J/mol K)
298 313 328
6.641 6.974 7.068
− 4.694 − 5.057 − 5.336
1.704
21.50
Acknowledgments This work was supported by the Postdoctoral Innovation Fund of Shandong Province (200802020), National Natural Science Foundation of China (50878121, 21007034), the Key Projects in the National Science & Technology Pillar Program in the Eleventh Five-year Plan Period, (No. 2006BAJ08B05), and the Science and Technology Development Key Program of Shandong Province, China (No. 2006GG2206007). References [1] T. Robinson, G. MucMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [2] Y. Yu, Y.Y. Zhuang, Z.H. Wang, M.Q. Qiu, Adsorption of water-soluble dyes onto modified resin, Chemosphere 54 (2004) 425–430. [3] M. Constapel, M. Schellenträger, J.M. Marzinkowski, S. Gäb, Degradation of reactive dyes in wastewater from the textile industry by ozone: analysis of the products by accurate masses, Water Res. 43 (2009) 733–743. [4] K.P. Katuri, S.V. Mohan, S. Sridhar, B.R. Pati, P.N. Sarma, Laccase-membrane reactors for decolorization of an acid azo dye in aqueous phase: process optimization, Water Res. 43 (2009) 3647–3658. [5] J. Sarasa, M.P. Roche, M.P. Ormad, E. Gimeno, A. Puig, J.L. Ovelleiro, Treatment of a wastewater resulting from dyes manufacturing with ozone and chemical coagulation, Water Res. 32 (1998) 2721–2727. [6] G.M. Walker, L.R. Weatherley, Fixed bed adsorption of acid dyes onto activated carbon, Environ. Pollut. 99 (1998) 133–136. [7] R. Marangonia, M. Bouhentb, C. Taviot-Guéhoc, F. Wypycha, F. Leroux, Zn2Al layered double hydroxides intercalated and adsorbed with anionic blue dyes: a physico-chemical characterization, J. Colloid Interface Sci. 333 (2009) 120–127. [8] D. Mohan, K.P. Singh, Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse — an agricultural wastes, Water Res. 36 (2002) 2304–2318. [9] U. Kumar, M. Bandyopadhyay, Sorption of Cd from aqueous solution using pretreated rice husk, Bioresour. Technol. 97 (2006) 104–109. [10] A. Khaled, A.E. Nemr, A. El-Sikaily, O. Abdelwahab, Removal of direct N Blue-106 from artificial textile dye effluent using activated carbon from orange peel: adsorption isotherm and kinetic studies, J. Hazard. Mater. 165 (2009) 100–110. [11] M. Özacar, İ.A. Şengil, A kinetic study of metal complex dye sorption onto pine sawdust, Process Biochem. 40 (2005) 565–572. [12] U.S. Orlando, A.U. Baes, W. Nishijima, M. Okada, Preparation of agricultural residue anion exchangers and its nitrate maximum adsorption capacity, Chemosphere 48 (2002) 1041–1046. [13] B.H. Hameed, D.K. Mahmoud, A.L. Ahmad, Sorption of basic dye from aqueous solution by pomelo (Citrus grandis) peel in a batch system, Colloids Surf., A 316 (2008) 78–84. [14] T. Robinson, B. Chandran, P. Nigam, Removal of dyes from a synthetic textile dye effluent by biosorption on apple pomace and wheat straw, Water Res. 36 (2002) 2824–2830.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Preparation, characterization of modified wheat residue and its utilization for the anionic dye removal Qian-Qian Zhong, Qin-Yan Yue ⁎, Qian Li, Xing Xu, Bao-Yu Gao Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
a r t i c l e
i n f o
Article history: Received 27 April 2010 Received in revised form 15 September 2010 Accepted 15 September 2010 Available online 12 October 2010 Keywords: Sorption Kinetics Equilibrium isotherm Dye Modified wheat residue (MWR)
a b s t r a c t The utilization of modified wheat residue (MWR) as sorbent to remove the anionic dye (Reactive Red-24, RR24) from aqueous solution was studied. MWR was prepared and characterized by specific surface area, SEM, zeta potential, FTIR and elemental analysis. Sorption experiments were carried out as a function of sorbent dosage, pH, contact time and concentration of dye. Results indicated that a mass of amine groups were grafted into the framework of MWR. It was shown that pseudo-second-order kinetic equation could best describe the sorption kinetics. The equilibrium sorption data were well represented by the Langmuir isotherm equation. The maximum sorption capacity of MWR for RR-24 was 200.0 mg/g, which showed higher capacity than those of previous work and a similar capacity compared to those of commercial activated carbon. The results indicated that MWR could be employed as an excellent and low-cost sorbent for removal of anionic dye from aqueous solution. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The textile industry produces large amount of colored effluents, in which dyes are highly visible and can be carcinogens and toxic to aquatic life in water. Reactive dyes have been identified as the most problematic compounds in textile dye effluents, as they tend to pass through conventional treatment systems unaffected. Conventional biological treatment processes are not very effective for their high solubility and non-biodegradability [1,2]. Therefore many methods, such as coagulation–flocculation, chemical oxidation, membrane processes, and adsorption [1,3–7], are used for the removal of the dyes. Of all these, activated carbon is the most commonly used sorbent for the treatment of dye bearing wastewaters. However, this process is proved to be uneconomical due to the high cost of activated carbon and also the additional cost involved in regeneration [8]. Therefore, further development of sorbents has been investigated, which focuses on the research of sorbents prepared from agricultural residues (AR), including bagasse [8], rice husk [9], orange peel [10], pine sawdust [11], coconut husk [12], pomelo peel [13] and apple pomace [14]. In North China, wheat is a very common and abundant crop and its by-product wheat residue (WR) is being considered as a significant waste disposal problem nowadays. The idea of converting wheat residue into a sorbent is based on the predominant composition of cellulose (32.1%), hemicellulose (29.2%), lignin (16.4%) and extrac-
⁎ Corresponding author. Tel.: + 86 531 88365258; fax: + 86 531 88364513. E-mail address: [email protected] (Q.-Y. Yue). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.025
tives (22.3%) in WR [15–17]. Due to a large amount of easily accessible hydroxyl groups that exist in cellulose, hemicellulose and lignin structures, a series of chemical reactions, such as condensation, etherification and copolymerization can easily happen [17,18]. In the present work, a new sorbent based on WR is present, which comprises first of etherification at alkaline condition, then an introduction of amine groups into the framework of WR. MWR bearing amine groups was characterized in relation to its physicochemical structure and then used as sorbent for the removal of anionic dye (RR-24) from aqueous solution. Batch studies were performed to evaluate the effect of various experimental parameters on the removal of RR-24. In addition, kinetic studies were carried out taking the initial dye concentration into account. The sorption capacities of MWR for RR-24 were investigated by the equilibrium isotherms. Thermodynamics of sorption process were studied and the changes in Gibbs free energy, enthalpy and entropy of sorption were also determined. 2. Materials and methods 2.1. Preparation of MWR Biomass WR, collected from Liao Cheng, Shandong, China, was washed with tap water followed by distilled water, and oven dried at 105 °C for 24 h, and then sieved, finally the particles ranging from 150 to 380 μm were selected for further chemical modification. Six grams of WR was dispersed in 100 mL of 15%NaOH (w/w) in a 500 mL three-neck round bottom flask and stirred for 1 h at room temperature followed by adding 40 mL of epichlorohydrine and the mixture was stirred for 3.5 h at 30 °C. The reaction product was
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CI
washed with distilled water to remove residual chemicals, and dried at 60 °C in a vacuum drier for 12 h. The dried product was then reacted with 50 mL 40% dimethylamine solution (w/w) for 5 h at 30 °C. The reaction product was washed twice with distilled water until the eluant was neutral and then dried at 60 °C in a vacuum drier for 12 h. The final product MWR was obtained and used in all sorption experiments. The chemical reactions using cellulose as a starting material are shown in Fig. 1.
N
SO3Na OH
NH
N
N
N N NH SO3Na
NaO3S
2.2. Preparation of dye solution
Cl
Fig. 2. Chemical structure of RR-24 (chemical formula: C25H14N7Cl2O10S3Na3 molecular weight: 808.5).
RR-24, obtained from Bin Zhou Dye Printing Co. (Shandong, China), in commercial purity, was used without further purification. The chemical structure of RR-24 is shown in Fig. 2. The dye stock solutions were prepared by dissolving accurately weighted dyes in distilled water to the concentration of 1000 mg/L and subsequently diluted when necessary.
and coated with platinum in vacuum evaporator before observation. The accumulation voltage and current were 3.0 kV and 10 μA, respectively. Zeta potential measurements were carried out using a microelectrophoresis apparatus (JS94H, Shanghai Zhongchen Digital Technical Apparatus Co., Ltd, China) to determine the zeta potentials of MWR and WR. FTIR spectra were recorded on an Avatar 370 spectrometer (Thermo Nicolet, USA) to investigate the functional groups present in MWR. For the spectra, 5 mg of samples was encapsulated in 400 mg of spectroscopically pure KBr and the specimens were pressed into small translucent discs. IR spectra were obtained by averaging 60 scans from 4000–400 cm− 1 region at 2 cm− 1 resolution.
2.3. Characterization of MWR Specific surface area measurements were performed with an automatic BET surface area analyzer (Model F-Sorb 2400, Beijing Jinaipu Technical Apparatus Co., Ltd, China). The detection limit of this instrument, using N2, is 0.01 m2 g− 1. SEM of the sample was obtained by Scanning electron microscope JSM-7600 F, JEOL, Japan). The samples were mounted on metal grids
OH H
OH
CH2OH O
O
H O CH2OH
H
OH
CH2ONa
H
O
OH
OH
CH
CH2
OH
CH H
H OH CH2
CH2O CH2 CH
CH2
+
O
CH3
(Dimethylamine)
HN
OH
CH3
O
OH OH
H
CH2O CH2
CH
O O
H O H
CI
(Epichlorohydrine)
CH2O CH2 H
O
OH
CH2
O H
H H
+
O
OH
O O
H
H
H
O CH2ONa
H
NaOH
(Cellulose)
O O
+
O
OH
OH H
OH
H
H
H
OH
H H OH
+ CH2OCH2 CH CH2 N H CI
H
CH2
+ NH CI
O
OH CH3 CH3
Fig. 1. Surface chemical modification of cellulose.
CH3 CH3
Q.-Q. Zhong et al. / Desalination 267 (2011) 193–200
The nitrogen content of MWR was measured by element analyzer (Elementar Vario EL III, Germany) to evaluate the grafted amine groups in the MWR.
195
The amount of adsorbed dye qt (mg/g) at different time t, was calculated as follows: qt =
ðC0 −Ct ÞV m
ð1Þ
2.4. Sorption experiments Sorption experiments for MWR were carried out in a batch equilibrium technique. The pH of the dye solution was adjusted using HCl or NaOH (0.1 mol/L). The sorption of dye was performed by shaking a predetermined amount of MWR in a 100 mL synthetic dye solution (with known initial dye concentration) at 140 rpm on a horizontal shaker. The solutions initial pH value (pH = 4.6) was used throughout all sorption experiments. After a given contact time for sorption, the solution was filtered and then analyzed quantitatively. The residual dye concentration in the clear supernatant was then measured with UV–visible spectrophotometer (model UV754GD, Shanghai) at the wavelength corresponding to a maximum absorbance, λmax, which is 534 nm for RR-24.
where C0 (mg/L) and Ct (mg/L) are the initial concentration of dye and the concentration at time t, respectively. V (l) stands for the volume of solution, and m (g) is the weight of the sorbent. The dye removal efficiency, η, was calculated by the following equation: ηð%Þ =
C0 −Ct × 100 C0
ð2Þ
3. Results and discussion 3.1. Characterization Data of specific surface areas of MWR and WR are only 4.8 and 6.1 m2 g− 1, respectively. A slight decrease in specific surface area of MWR is observed after the chemical modification; this result indicates that the extractives have been gotten rid of from the surface of MWR after the chemical modification. The low specific surface area of MWR illustrates the fact that MWR does not have a similar porous structure in active carbon (specific surface area higher than 500 m2 g− 1), and therefore, surface sorption will be absent in the potential sorption mechanism for RR-24 sorption onto MWR. The SEM images of the structure of WR and MWR (Fig. 3) show that the surface of MWR is smoother than that of WR, which validates the lower specific surface area of MWR in comparison with that of WR. Similar results were reported in the works of Xu and Wang for the preparation of anionic absorbents from agricultural residues [16,19]. The zeta potentials of WR are in the range of −35 to −30 mV. The great negative zeta potential of WR can be attributed to the large amounts of hydroxyl and carboxyl groups existing in WR. In contrast with the WR, the zeta potential of MWR is in the range of −10 to 0 mV; this result indicates the existence of positive-charge functional groups in the framework of MWR, which results in the significant decrease in the negative charge of MWR. The FTIR spectra of WR and MWR are presented in Fig. 4. The band intensity near 3390 cm− 1 indicates a mass of O–H stretching vibration, and the band position at 2900 cm− 1 is associated with C– H stretching vibration [20]. The bands observed in the 1625 cm− 1 is due to O–H bending vibration of water molecules, which is consistent
(a)
Transmittance (a.u.)
(b)
WR MWR
500
1000
1500
2000
2500
3000
Wavenumber(cm-1) Fig. 3. SEM images of WR (a) and MWR (b).
Fig. 4. FTIR analysis of WR and MWR.
3500
4000
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with the work of Marangonia [7]. The FTIR analysis of MWR displays the relative changes in the structure compared with that of WR. The band at 1630 cm− 1 is attributed to the N–H stretching [21], and an intense vibration is observed in the band at 1410 cm− 1, which corresponds to characteristic for C–N stretching vibration. These obviously come from amine groups grafted into the structure of the MWR. Similar results were also observed in Mishra and Gurgel's work [21,22]. Table 1 displays the elemental changes of carbon, hydrogen, and nitrogen in MWR in comparison with those of WR. A slight increase is observed in the carbon content and hydrogen content. Nitrogen content of MWR, however, increases significantly from 0.35% to 1.04%, indicating a three fold increase compared to WR. Therefore, it is possible to conclude that the reactions proceeded efficiently and amine groups have been anchored onto the MWR. Based on the characterization of MWR mentioned earlier, it is clear that the MWR has been grafted with a mass of functional groups which would be beneficial to the sorption of anionic dyes from solution.
Dye removal efficiency(%)
100
80
60 50mg/L dye + MWR 100mg/L dye + MWR
40
150mg/L dye + MWR 200mg/L dye + MWR 100mg/L dye + WR
20
0 0.4
0.8
1.2
1.6
2.0
Sorbent dosage(g/L) Fig. 5. Effect of sorbent dosage on sorption of RR-24 by MWR and WR (T = 26 °C, t = 4 h, initial pH).
3.3. Effect of pH on dye removal The pH of the solution plays an important role in the sorption capacity of sorbate molecule largely due to its influence on the surface characteristics of the sorbent and ionization/dissociation of the sorbate molecule [10]. Fig. 6 presents the effect of pH on dye removal using MWR. It is evident that dye uptake is higher at lower pH and as the pH of the solution increases, dye uptake decreases sharply. The RR-24 dye is reactive dyestuff, which contains –SO− 3 groups in its structure. Under acidic pH, the protonation of –SO− 3 groups enhances, and then more –SO3Na of dye molecule exists in the form of –SO3H, which can react with the amidocyanogens of MWR to + − − form –NH+ 2 SO3 or = NH SO3 [23]. As a result, the sorption of RR-24 onto MWR is significantly enhanced. However, as the pH of the system increases, the protonation of –SO− 3 decreases, and consequently – SO3H groups are reduced, resulting in the decrease of interaction between the dye molecules and MWR. Moreover, lower sorption of the anionic dye at alkaline pH is provable attributed to the abundance of OH− ions, which will compete with the dye anions for the sorption sites [10,24,25]. Similar observations have been reported by previous work for sorption of reactive dyes on activated carbon prepared from sugarcane bagasse pith, coir pith and orange peel [10,26,27].
The relationship between the initial and final pH for dye sorption indicates that MWR has a high pH buffering nature (Fig. 6). At lower pH, due to more H+ combined with –SO− 3 groups, the final pH elevates, resulting in the buffering of H+ by OH−. Similarly under alkaline pH, the final pH decreases compared with the initial pH, because of the sorption of OH− from the solution by MWR.
3.4. Effect of contact time and initial concentration The effects of initial dye concentration and contact time on sorption of RR-24 onto MWR are investigated in Fig. 7 (a). As shown in Fig. 7 (a), the amount qt of RR-24 sorbed onto MWR increases from 41.5 to 118.9 mg/g as the initial concentration of dye increases from 50 to 150 mg/L, attributed to the increase in the driving force of the concentration gradient with the increase in the initial dye concentration [28]. The contact time is an important parameter in the wastewater treatment by sorption. It is observed that the contact time increases with an increase of initial concentration of the dye. The results show that the contact time needed for RR-24 solutions with initial concentrations of 50–100 mg/L to reach equilibrium are within 60 min. For RR-24 solution with initial concentration of 150 mg/L, equilibrium time of 240 min is required. It may be due to the increase
100
12
80
10
8
60
6
Final pH
The effect of sorbent dosage on RR-24 removal is shown in Fig. 5. It is observed that the removal efficiency of dye increases up to a maximum efficiency as the dosage of MWR increases, probably due to the increased availability of more sorption sites with the increase in MWR dosage. For the concentration in the range of 50–150 mg/L dye solutions treated using 1.2 g/L MWR, all the dye removal efficiencies exceed 92.2%; meanwhile, for dye concentration of 200 mg/L, 95.7% of the removal efficiency is obtained using MWR dosage of 2.0 g/L. Fig. 5 also illustrates the RR-24 removal capacity of WR. It is clearly seen that RR-24 is hardly absorbed by WR. The results probably indicate that a chemical modification is required to introduce some functional groups into the structure of WR for the dye removal. A similar conclusion was observed in the adsorption of phosphate onto modified wheat straw prepared by triethylamine [17].
Dye removal efficiency(%)
3.2. Effect of sorbent dosage on dye removal
40 4 20 2
Table 1 Change of element content of WR and MWR.
0
Materials
N (%)
C (%)
H (%)
WR MWR
0.35 1.04
41.11 43.58
6.10 6.64
2
4
6
8
10
12
initial pH Fig. 6. Effect of initial pH on decolorization and final pH (MWR dosage 1.2 g/L, initial RR24 concentration 100 mg/L, t = 26 °C, t = 4 h).
Q.-Q. Zhong et al. / Desalination 267 (2011) 193–200
197
(b) pseudo first-order
(a)
6
120 4 100
ln(qe-qt)/mg g-1
qt/ mg g-1
2 80
60
40
0 -2 -4
50mg/L
50mg/L 80mg/L
80mg/L
20
-6
100mg/L
100mg/L
150mg/L
0
50
0
100
150
200
150mg/L
-8
250
0
50
100
t (min)
150
200
t (min)
(c) pseudo second-order
(d) intra-particle equation
6
120 50mg/L
5
80mg/L
100
150mg/L
4
qt/ mg g-1
t/qt /min g mg-1
100mg/L
3
80
60
2 40 50mg/L
1
80mg/L
20
100mg/L 150mg/L
0 0
50
100
150
200
0
250
0
2
4
6
8
10
12
14
16
t1/2 (min)
t (min)
Fig. 7. Effect of contact time on the uptake of RR-24 by MWR at different initial concentrations (a) and kinetics models (b, c, d) (T = 26 °C, MWR dosage 1.2 g/L, initial pH).
in dye concentration, the competition for the active sorption sites increases and it takes longer time to reach equilibrium [29]. It is observed from Fig. 7 (a) that the sorption process of RR-24 onto MWR mainly consists of two stages: initial rapid stages and gradual sorption stages. The high sorption rate during the initial period is due to a number of available sorption sites of the bare surface of sorbent. As these sites become progressively covered, the rate of sorption decreases [30]. 3.5. Sorption kinetics In order to investigate the controlling mechanisms of sorption process such as chemical reaction, mass transfer and diffusion control, several kinetic models are required to analyze the experimental data [10].Three kinetic models, namely pseudo-first-order, pseudo-second-order and intra-particle diffusion models are used for the sorption of RR-24 onto MWR. The pseudo-first-order equation [30,31] is given as Eq. (3): ln ðqe −qt Þ = ln qe −k1 t
ð3Þ
where qe and qt (mg/g) are the amounts of dyes sorbed per unit weight of sorbent at equilibrium and time t (min), respectively;
k1 (min− 1) is the rate constant of pseudo-first-order sorption. The pseudo-second-order kinetic rate equation [32,33] is expressed as Eq. (4): t 1 t = + qt qe k2 qe2
ð4Þ
where k2 (g/(mg min)) is the equilibrium rate constant of pseudosecond-order sorption. The initial rate of intra-particle equation can be described as Eq. (5) [17,33]: qt = kp t
0:5
+C
ð5Þ
where kp (g mg− 1 min− 0.5) is the intra-particle rate constant, which can be evaluated from the slope of the linear plot of qt versus t0.5; C is the intercept, related to the thickness of the boundary layer. The kinetic parameters at different initial concentrations are calculated and correlation coefficients (R2) are evaluated as shown in Fig. 7 (b, c, d) and Table 2. Results shown in Fig. 7 indicate that the pseudo-second-order kinetic model can be applied for the entire sorption period. The values of R2 (Table 2) of pseudo-second-order model for RR-24 sorption are satisfactory (N0.9989) and followed by those of modified pseudo-
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Table 2 Kinetic parameters for sorption rate expressions. C mg/L 50 80 100 150
qe expt mg/g
Pseudo-first-order
41.66 66.61 82.98 118.8
Pseudo-second-order
Intra-particle diffusion
k1 min− 1
qe,cal mg/g
R2
k2 g/mg min
qe,
0.1103 0.0389 0.02978 0.02106
17.86 23.47 32.09 76.96
0.9661 0.9690 0.9424 0.9706
0.02338 0.005319 0.002721 0.000647
41.81 67.52 84.67 124.2
cal
mg/g
R2
kp mg/(g min)
C
R2
0.9999 0.9999 0.9999 0.9989
7.657 7.998 9.969 10.87
9.177 21.26 17.75 16.79
0.9062 0.8263 0.9329 0.9448
Where qe,expt is experimental value, qe,cal is calculated value.
first-order equation and intra-particle diffusion equation, respectively; this result indicates that the pseudo-second-order kinetic model generates the best agreement with the RR-24-MWR sorption system. Similar kinetic result was reported for the sorption of Direct N Blue106 onto activated carbon from orange peel [10]. According to Eq. (5), if the plot of qt versus t0.5 passes through the origin, intra-particle diffusion will be the rate-controlling step [10]. However, it is not the case in Fig. 7 (d), and therefore, intra-particle diffusion is not the sole rate-limiting step but some degree of boundary layer diffusion also controls the sorption [23]. Moreover, other sorption kinetic processes occur simultaneously during the RR24–MWR interactions and contribute to the sorption mechanism [10,17]. 3.6. Sorption isotherms The sorption isotherm of RR-24 onto MWR is illustrated in Fig. 8. The batch experimental curves are applied to three sorption isotherm models which are Langmuir, Freundlich, and Dubinin–Radushkevich (D–R).The models are represented mathematically as follows [34,35]: Langmuir equation: 1 1 = + qe qmax
1
1 qmaw KL Ce
ð6Þ
Freundlich equation: ln qe = ln KF +
1 ln Ce n
ð7Þ
D–R equation: 2
ln qe = ln qm −βε
ð8Þ
where qe (mg/g) and Ce (mg/L) are the sorbate equilibrium concentrations in solid and the liquid phases; qmax(mg/g), the
1 E = pffiffiffiffiffiffiffiffiffiffiffiffi −2β
ð9Þ
The theoretical parameters of isotherms along with regression coefficient are listed in Table 3. The Langmuir model yields the best fit (R2 = 0.989–0.995) with compared to the other isotherm models, which shows the homogeneous nature of the sorbent. Based on the Langmuir equation, the value of qmax increases with the temperature increasing, and reaches 200.0 mg/g at 55 °C, which confirms that the sorption process for RR-24 onto MWR is an endothermic reaction. The n values of Freundlich model are in the range of 2–10, indicating a favorable sorption process [34]. The E values of D–R isotherm obtained are lower than 8 kJ/mol; this indicates that the physical interaction is the process involved for adsorption of RR-24 onto MWR, similar observation was reported for sorption of Cr (VI) onto carbonaceous adsorbents prepared from waste biomass [35]. Table 4 lists the comparison of maximum sorption capacity of some dyes on various sorbent. Although the published values are obtained under different experimental conditions, they still will be useful as a criterion for comparing the sorption capacities. As can be seen from Table 4, the maximum sorption capacity of MWR for RR-24 is higher than those in most of previous studies and similar to those of commercial activated carbons; this indicates that MWR has a certain potential for practical application in dye removal from wastewater. 3.7. Sorption thermodynamic In engineering practice, thermodynamic parameters are crucial and must be taken into consideration in order to determine the spontaneity of a process, including the free energy changes (ΔG0), standard enthalpy changes (ΔH0) and the entropy changes (ΔS0) associated with the sorption process [30]. It can be calculated by the following equations [43]:
200
150
qe(mg/g)
monolayer capacity of the sorbent; KL (L/mg) is a constant of the Langmuir isotherm. KF (L/g) is Freundlich constant, and 1/n is the heterogeneity factor. qm (mg/g) is the theoretical saturation capacity in D-R equation; β the constant related to the sorption energy (mol2/ kJ2); and ε the Polanyi potential is equal to RT ln(1 + 1/Ce); E (kJ/mol) provides information about chemical and physical sorption and can be determined according to Eq. (9).
0
ΔG = −RT ln K0
ð10Þ
100 25°C 40°C 55°C
50
0
ln K0 =
0
100
200
300
400
500
600
Ce(mg/L) Fig. 8. Sorption isotherms of RR-24 onto MWR (MWR dosage 1.2 g/L, initial pH).
ΔS0 ΔH 0 − R RT
ð11Þ
where KL is Langmuir equilibrium constant (L/mol), R is the universal gas constant, 8.314 J mol− 1 K− 1, T is the absolute temperature. The enthalpy change is determined by plotting lnK0 versus 1/T. K0 is the thermodynamic equilibrium constant, calculated with respect to temperature using the method of Khan and Singh [43] by plotting ln (qe/Ce) versus qe and extrapolating to qe = 0.
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199
Table 3 Parameters obtained from Freundlich, Langmuir and D–R models. T (°C)
Langmuir isotherm qmax (mg/g)
KL (L /mg)
R2
KF (L/g)
Freundlich isotherm n
R2
qm (mg/g)
Dubinin–Radushkevich isotherm β × 102 (kJ2/mol2)
E (KJ/mol)
R2
25 40 55
172.4 188.7 200.0
0.09236 0.07715 0.07205
0.9953 0.9925 0.9893
70.60 74.38 78.19
6.897 6.734 6.729
0.9092 0.9510 0.9745
140.2 154.4 160.9
7.42 5.48 4.35
2.596 3.021 3.390
0.9033 0.9193 0.8629
Table 4 Comparison of the qe of some dyes on various sorbents. Dyes Reactive Reactive Reactive Reactive Reactive RR-24 RR-24 RR-24 RR-24 RR-24 RR-24 RR-24
red 195 red 2 red 194 red 194 red 120
Sorbent
qmax (mg/g)
Reference
Wheat bran Metal hydroxide sludge Sorel's cement Pine-fruit shell Coconut tree flower carbon Aluminum magnesium mixed hydroxide Chitosan Sludge activated carbon Modified resin Commercial activated carbon (basic) Commercial activated carbon (acidic) MWR
119.1 62.5 120.89 20.8 181.9 657.5 245 37.3 177.1 242 ± 7 163 ± 2 200.0
[32] [36] [37] [38] [39] [34] [40] [41] [2] [42] [42] This work
The thermodynamic parameters are listed in Table 5. The ΔG0 is negative and changes with the rise in temperature, which suggests that the sorption process is spontaneous in the nature and the spontaneity increases with the rise in temperature. ΔH0 (1.704 kJ/ mol) of the system is negative and smaller than 40 kJ/mol, indicating that the sorption of RR-24 onto MWR is endothermic and physical in nature [21], which is also verified by the sorption isotherm studies. The positive value of ΔS0 implies increased randomness at the solid– solution interface with some structural changes during sorption process and an affinity of the sorbent for dye [37,44].
4. Conclusions This study shows that MWR is a promising sorbent for removal of anionic dye, RR-24, from aqueous solution. The ratios of RR-24 sorbed keep above 95% over a range from 50 to 200 mg/L of dye concentration when 2.0 g/L of sorbent is used. A decrease in the pH of solutions leads to a significant increase in the removal of dye RR-24 on MWR. The pseudo-second-order model provides the best correlation of the experimental data and intra-particle diffusion is involved in the sorption process, but it is not the sole rate-limiting step. The isothermal data indicate that the Langmuir model provides much closer fittings than those of Freundlich and D–R models. The saturated sorption capacity of MWR for RR-24 is 200.0 mg/g at 55 °C. Negative ΔG0 indicates spontaneous sorption of RR-24 dye onto MWR at the studied temperatures. The enthalpy change ΔH0 (1.704 kJ/mol) of the system is positive and smaller than 40 kJ/mol, indicating that the sorption of RR-24 onto MWR is endothermic and physical in nature. This work shows that the MWR can be used as an inexpensive and effective alternative sorbent for the purification of wastewater to remove reactive dye from dye wastewater. Table 5 Thermodynamic parameters for the sorption of RR-24. T (K)
Ko
ΔG0 (kJ/mol)
ΔH0 (kJ/mol)
ΔS0 (J/mol K)
298 313 328
6.641 6.974 7.068
− 4.694 − 5.057 − 5.336
1.704
21.50
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